they go by score. anything that scores points counts toward killstreaks. there's more ways to score in objective games. tdm is just about kills so big streaks take more work. hardline gives you more points. a blackbird gives 25 points for every kill your teammates get. kills from killstreaks stack at 25pts per kill. dogs are possible in tdm but barely.

we might feast in kill confirmed though:
50 per kill
100 for the tag of enemies you killed
25 for denies or a teammates' tag

if you start the game 10-0 and get all 10 tags you have 1500pts. add hardline and you got dogs. plus, unlike mw3, your last killstreak stacks to your first.